With the constantly improving quality of color photocopies and printings and in an attempt to protect security documents such as banknotes, value documents or cards, transportation tickets or cards, tax banderols, and product labels against counterfeiting, falsifying or illegal reproduction, it has been the conventional practice to incorporate various security means in these documents. Typical examples of security means include security threads or stripes, windows, fibers, planchettes, foils, decals, holograms, watermarks, security inks comprising optically variable pigments, magnetic or magnetizable thin film interference pigments, interference-coated particles, thermochromic pigments, photochromic pigments, luminescent, infrared-absorbing, ultraviolet-absorbing or magnetic compounds.
Security threads embedded in the substrate are known to those skilled in the art as an efficient means for the protection of security documents, in particular banknotes, against imitation. Reference is made to U.S. Pat. No. 0,964,014; U.S. Pat. No. 4,652,015; U.S. Pat. No. 5,068,008; U.S. Pat. No. 5,324,079; WO 90/08367 A1; WO 92/11142 A1; WO 96/04143 A1; WO 96/39685 A1; WO 98/19866 A1; EP 0 021 350 A1; EP 0 185 396 A2; EP 0 303 725 A1; EP 0 319 157 A2; EP 0 518 740 A1; EP 0 608 078 A1; and EP 1 498 545 A1 as well as the references cited therein. A security thread is a metal- or plastic-filament, which is incorporated during the manufacturing process into the substrate serving for printing security documents or banknotes. Security threads or stripes carry particular security elements, serving for the public- and/or machine-authentication of the security document, in particular for banknotes. Suitable security elements for such purpose include without limitation metallizations, optically variable compounds, luminescent compounds, micro-texts and magnetic features.
With the aim of protecting value documents such as banknotes from being forged, optically variable security threads or stripe exhibiting color shift or color change upon variation of the angle of observation have been proposed as security features to be incorporated into or onto said value documents. The protection from forgery is based on the variable color effect that optically variable security elements convey to the viewer in dependence on the viewing angle or direction. Reference is made to WO 2004/048120 A1, US 2007/0241553, WO 2007/042865, US 2011/0095518 and EP 2 465 701 A2 as well as the references cited therein.
In addition to static security features used for protecting security documents against counterfeit and illegal reproduction, dynamic security features providing the optical illusion of movement have been developed. In particular, security elements based on oriented magnetic or magnetizable pigments and magnetic or magnetizable optically variable pigments have been developed so as to provide an optical illusion of movement.
WO 2011/107527 A1 discloses thread and stripes comprising a plastic foil which carries a hardened coating comprising oriented magnetic or magnetizable pigment particles, in particular optically variable magnetic or magnetizable pigment particles, the orientation of said pigment particles representing graphic information.
U.S. Pat. No. 7,047,883 discloses the creation of a dynamic optically variable effect known as the “rolling bar” feature. The “rolling bar” feature provides the optical illusion of movement to images comprised of oriented magnetic or magnetizable pigments. U.S. Pat. No. 7,517,578 and WO 2012/104098 A1 respectively disclose “double rolling bar” and “triple rolling bar” features, said features seeming to move against each other upon tilting. A printed “rolling bar” type image shows one or contrasting bands which appear to move (“roll”) as the image is tilted with respect to the viewing angle. Such images are known to be easily recognized by the man on the street and the illusive aspect cannot be reproduced by commonly available office equipment for color scanning, printing and copying. “Rolling bar” features are based on a specific orientation of magnetic or magnetizable pigments. In particular, the magnetic or magnetizable pigments are aligned in a curving fashion, either following a convex curvature (also referred in the art as negative curved orientation) or a concave curvature (also referred in the art as positive curved orientation).
WO 2012/104098 A2 discloses a method for producing “triple rolling bar” features, said method comprising the steps of: a) applying a coating composition comprising magnetic or magnetizable pigment particles onto a substrate; b) orienting said magnetic or magnetizable pigment particles according to a first curved surface by applying a first magnetic field; c) selectively hardening said applied coating composition in first areas, hereby fixing the magnetic pigment particles in their positions and orientations; d) orienting said magnetic or magnetizable pigment particles in the unhardened part of the coating composition according to a second curved surface by applying a second magnetic field; e) hardening said applied coating composition in second areas, hereby fixing the magnetic pigment particles in their positions and orientations. For achieving an area comprising pigments particles oriented to follow a negative curvature and an area comprising pigments particles oriented to follow a positive curvature, the disclosed method requires on one hand to orient the pigments particles by applying a magnetic-field generating device from the bottom of the substrate and, on the other hand, by applying a magnet from the top of the substrate.
However, disclosed methods to obtain a security element comprising a substrate and combining at least two areas, one area comprising magnetic or magnetizable pigment particles oriented so as to follow a negative curvature and another area comprising magnetic or magnetizable oriented to follow a positive curvature require a step of applying a magnetic-field generating device from above the substrate, i.e. the device faces the not yet hardened composition comprising the magnetic or magnetizable pigment particles, therefore increasing the complexity of the overall manufacturing process of the security element. For example, the not yet hardened composition should not be placed in direct contact with the magnetic-field generating device so as to avoid or exclude any deterioration of the optical effect. Moreover, since the strength of a magnetic field decreases rapidly with distance, if the magnetic-field generating device is positioned at a large distance from the not yet hardened composition to avoid direct contact, the orientable pigment particles will be oriented by a weaker magnetic field resulting in a less striking optical effect.
A need remains for providing sophisticated security threads or stripes combining high visual attractiveness with a highly sophisticated design so as to further increase the resistance against counterfeiting or illegal reproduction of security documents comprising said security threads or stripes. A need also remains for a simpler and more efficient process for producing said sophisticated security threads or stripes.